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  <div class="section" id="storage-engine">
<h1>Storage Engine<a class="headerlink" href="#storage-engine" title="Permalink to this headline">¶</a></h1>
<div class="section" id="commitlog">
<span id="commit-log"></span><h2>CommitLog<a class="headerlink" href="#commitlog" title="Permalink to this headline">¶</a></h2>
<div class="admonition-todo admonition" id="index-0">
<p class="first admonition-title">Todo</p>
<p class="last">todo</p>
</div>
</div>
<div class="section" id="memtables">
<span id="id1"></span><h2>Memtables<a class="headerlink" href="#memtables" title="Permalink to this headline">¶</a></h2>
<p>Memtables are in-memory structures where Cassandra buffers writes.  In general, there is one active memtable per table.
Eventually, memtables are flushed onto disk and become immutable <a class="reference internal" href="#sstables">SSTables</a>.  This can be triggered in several
ways:</p>
<ul class="simple">
<li>The memory usage of the memtables exceeds the configured threshold  (see <code class="docutils literal"><span class="pre">memtable_cleanup_threshold</span></code>)</li>
<li>The <a class="reference internal" href="#commit-log"><span class="std std-ref">CommitLog</span></a> approaches its maximum size, and forces memtable flushes in order to allow commitlog segments to
be freed</li>
</ul>
<p>Memtables may be stored entirely on-heap or partially off-heap, depending on <code class="docutils literal"><span class="pre">memtable_allocation_type</span></code>.</p>
</div>
<div class="section" id="sstables">
<h2>SSTables<a class="headerlink" href="#sstables" title="Permalink to this headline">¶</a></h2>
<p>SSTables are the immutable data files that Cassandra uses for persisting data on disk.</p>
<p>As SSTables are flushed to disk from <a class="reference internal" href="#memtables"><span class="std std-ref">Memtables</span></a> or are streamed from other nodes, Cassandra triggers compactions
which combine multiple SSTables into one.  Once the new SSTable has been written, the old SSTables can be removed.</p>
<p>Each SSTable is comprised of multiple components stored in separate files:</p>
<dl class="docutils">
<dt><code class="docutils literal"><span class="pre">Data.db</span></code></dt>
<dd>The actual data, i.e. the contents of rows.</dd>
<dt><code class="docutils literal"><span class="pre">Index.db</span></code></dt>
<dd>An index from partition keys to positions in the <code class="docutils literal"><span class="pre">Data.db</span></code> file.  For wide partitions, this may also include an
index to rows within a partition.</dd>
<dt><code class="docutils literal"><span class="pre">Summary.db</span></code></dt>
<dd>A sampling of (by default) every 128th entry in the <code class="docutils literal"><span class="pre">Index.db</span></code> file.</dd>
<dt><code class="docutils literal"><span class="pre">Filter.db</span></code></dt>
<dd>A Bloom Filter of the partition keys in the SSTable.</dd>
<dt><code class="docutils literal"><span class="pre">CompressionInfo.db</span></code></dt>
<dd>Metadata about the offsets and lengths of compression chunks in the <code class="docutils literal"><span class="pre">Data.db</span></code> file.</dd>
<dt><code class="docutils literal"><span class="pre">Statistics.db</span></code></dt>
<dd>Stores metadata about the SSTable, including information about timestamps, tombstones, clustering keys, compaction,
repair, compression, TTLs, and more.</dd>
<dt><code class="docutils literal"><span class="pre">Digest.crc32</span></code></dt>
<dd>A CRC-32 digest of the <code class="docutils literal"><span class="pre">Data.db</span></code> file.</dd>
<dt><code class="docutils literal"><span class="pre">TOC.txt</span></code></dt>
<dd>A plain text list of the component files for the SSTable.</dd>
</dl>
<p>Within the <code class="docutils literal"><span class="pre">Data.db</span></code> file, rows are organized by partition.  These partitions are sorted in token order (i.e. by a
hash of the partition key when the default partitioner, <code class="docutils literal"><span class="pre">Murmur3Partition</span></code>, is used).  Within a partition, rows are
stored in the order of their clustering keys.</p>
<p>SSTables can be optionally compressed using block-based compression.</p>
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